Voltage controlled oscillators (VCOs) are commonly employed in a variety of applications, including communication and timing circuitry. In particular, voltage controlled oscillators are commonly used in phase locked loop control systems. Functionally, a voltage controlled oscillator may be viewed as a circuit that seeks to transform an input control voltage signal to an output signal having a desired frequency.
Referring to FIG. 1, a clock diagram of a phase lock loop voltage controlled oscillator (PLL VCO) 100 in accordance with the conventional art is shown. As depicted in FIG. 1, the PLL VCO 100 comprises a reference clock 110, a phase/frequency detector 115, a charge pump 120, a voltage controlled oscillator (VCO) 125, and a feedback divider 130.
The phase-frequency detector 115 is responsive to a reference clock 150 and a feedback clock 155. The phase-frequency detector 115 may provide one or more out-of-lock indicator signals corresponding to whether and to what extent the feedback clock 155 is out-of-lock with the reference clock 150. In one implementation, the out-of-lock indicator signals may be up and down signals 160, 165. The charge pump 120 is responsive to the out-of-lock indicator signals, and provides for generating a control signal 170 whose voltage potential corresponds to a phase error between the feedback clock 155 and the reference clock 150. The VCO 125 is responsive to the control signal 170, and provides for generating an output signal 175 having a desired frequency (e.g., 2.44 GHz).
The feedback divider 130 reduces the frequency of the output signal 175. The divide value (e.g., N=2400) is selected based upon the frequency of the reference clock 150 and the frequency of the desired output signal 175. When the output signal 175 is operating at the desired frequency, the feedback divider 130 reduces the frequency of the output signal 175 such that the frequency of the feedback clock 155 is equal to the frequency of the reference clock 150 (e.g., 1 MHz), and hence the PLL VCO 100 operates in a phase lock condition. If the frequency of the output signal 175 is higher than desired, the frequency of the feedback clock signal 155 will be greater than the frequency of the reference clock 150. As a result the phase/frequency detector 115 will output a down signal 165 causing the control signal 170 from the charge pump 120 to be decreased. Thus, the oscillating frequency of the VCO 125 will be reduced, until the phase lock condition is achieved again. If the frequency of the output signal 175 is lower than desired, the frequency of the feedback clock signal 155 will be less than the frequency of the reference clock 150. As a result the phase/frequency detector 160 will output an up signal 160, causing the control signal 170 from the charge pump 120 to be increased. Thus, the oscillating frequency of the VCO 125 will be increased, until the phase lock condition is achieved again.
The VCO 125 operates within a finite band of frequencies (e.g., 2.1 to 2.9 GHz). Ideally, this band would be centered around a desired frequency. However, process variations limit the accuracy that can be achieved in centering the frequency band around the desired frequency. The operating temperature also causes variations of the operating band of the voltage controlled oscillator circuit.
One solution, according to the conventional art, is to provide additional circuitry to expand the frequency band to ensure that all frequencies are available to compensate for process and temperature variations. This method is disadvantageous in that it adds additional cost and uses additional valuable real estate. The additional tuning range is also disadvantageous in that it causes the voltage controlled oscillator 125 to be more sensitive to noise and interference.
Another solution, according to the convention art, is to manually adjust the frequency band to be centered about the desired operating frequency. However, manual adjustments are an added cost, and require the customer or user to correctly perform the operation. This can lead to performance degradation and can be tedious and/or error prone.